Chylothorax is a rare, but serious complication in children who undergo heart surgery, and is an important factor for the length of stay in pediatric intensive care units (PICU). Incidence of these pleural effusions following cardiac surgery in children is estimated to be 1.3 - 9.2% and has increased significantly in the last two decade. Chylo- and lympho-thorax can lead to serious metabolic, immunologic, and nutritional complications, and is associated with high morbidity and mortality rate. The pathogenesis is poorly understood but is thought to arise from (i) direct damage to the thoracic duct by rupture, laceration, tear, or compression, (ii) central venous (CV) obstruction or high pressure that impedes thoracic duct drainage into the venous system, or (iii) smaller lymphatic defects that may accompany congenital heart defects. Treatments for effective management need to be based upon known mechanism, but there is no diagnostic method to interrogate the causes of pleural effusion in any individual case. Currently, management for pleural effusions is initially conservative, consisting of gut rest as well as dietary modification over a period of 3-4 weeks. Although 75 to 85% of patients respond to conservative treatment, last resort surgical treatments such as pleurodesis and thoracic duct ligation have only limited success in treating pleural effusions, probably due to their lateness of treatment. Improving the poor response to these therapies demands a better understanding of the specific lymphatic drainage mechanisms responsible. Yet the inability to dynamically image lymphatic drainage in infants within the PICU has limited the understanding of pathogenesis of pleural effusions, which in turn has impeded the application of appropriate and timely management for these critically ill children. Near-infrared fluorescence (NIRF) imaging has been successfully used in investigational studies to assess the lymphatic vasculature and its function in adults and more recently in an infant with chylothorax. In this application, we propose to conduct a pilot study of dynamic lymphatic imaging of young children and infants who present with pleural effusion. Our specific aims of the study will be (i) to adapt our current investigational NIRF imaging device to enable ambient light rejection for improved bedside application in the PICU; (ii) to use the modified device to characterize dynamic lymphatic drainage in children with pleural effusion to assess drainage pathways; and (iii) to monitor response to therapy, whether conservative treatment, pleurodesis, or thoracic duct ligation in order to gain a better understanding of the management of chylothorax.